docs: def relevant_connections(n, _from, to):
      """Construct a connectivity matrix.
  
      Args:
          n (int): The dimensions of the matrix
          _from (tuple[int]): Nodes with outgoing connections to ``to``
          to (tuple[int]): Nodes with incoming connections from ``_from``
  
      Returns:
          np.ndarray: An |n x n| connectivity matrix with the |i,jth| entry is
          ``1`` if |i| is in ``_from`` and |j| is in ``to``, and 0 otherwise.
      """
      cm = np.zeros((n, n))
  
      # Don't try and index with empty arrays. Older versions of NumPy
      # (at least up to 1.9.3) break with empty array indices.
      if not _from or not to:
          return cm
  
      cm[np.ix_(_from, to)] = 1
      return cm